Infinite Series Problem

[chazzs]

So far none of the suggestions have worked, neither with the summation sign, nor the sum command. I think the problem lies with it being an alternating series and the parsing of the parentheses. The prime seems to be ignoring the parentheses around the base (-0.75)^k. Non alternating series and partial sums seem to work, but not an infinite alternating series. Still not sure what is actually happening though. Seems a little random on what is evaluated and what is no.